Rabbit 6000 User’s Manual
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207
Several types of data encoding are available in the HDLC mode. In addition to the normal NRZ,
they are NRZI, biphase-level (Manchester), biphase-space (FM0), and biphase-mark (FM1).
Examples of these encodings are shown below. Note that the signal level does not convey infor-
mation in NRZI, biphase-space, and biphase-mark. Instead it is the placement of the transitions
that determine the data. In biphase-level it is the polarity of the transition that determines the data.
Figure 20-1. Examples of Data Encoding In the HDLC Mode
In the HDLC mode the internal clock comes from the output of Timer A2/Timer A3 or the dedi-
cated divider. The timer/divider output is divided by 16 to form the transmit clock, and is fed to
the digital phase-locked loop (DPLL) to form the receive clock. The DPLL is basically just a
divide-by-16 counter that uses the timing of the transitions on the receive data stream to adjust its
count. The DPLL adjusts the count so that the DPLL output will be properly placed in the bit cells
to sample the receive data. To work properly, then, transitions are required in the receive data
stream. NRZ data encoding does not guarantee transitions in all cases (a long string of zeros, for
example), but the other data encodings do. NRZI guarantees transitions because of the inserted
zeros, and the biphase encodings all have at least one transition per bit cell.
The DPLL counter normally counts by 16, but if a transition occurs earlier or later than expected,
the count will be modified during the next count cycle. If the transition occurs earlier than
expected, it means that the bit cell boundaries are early with respect to the DPLL-tracked bit-cell
boundaries, so the count is shortened by either one or two counts. If the transition occurs later
than expected, it means that the bit-cell boundaries are late with respect to the DPLL-tracked bit-
cell boundaries, so the count is lengthened by either one or two counts. The decision to adjust by
one or by two depends on how far off the DPLL-tracked bit cell boundaries are. This tracking
allows for minor differences in the transmit and receive clock frequencies.
With NRZ and NRZI data encoding, the DPLL counter runs continuously, and adjusts after every
receive data transition. Since NRZ encoding does not guarantee a minimum density of transitions,
the difference between the sending data rate and the DPLL output clock rate must be very small,
and depends on the longest possible run of zeros in the received frame. NRZI encoding guarantees
at least one transition every six bits (with the inserted zeros). Since the DPLL can adjust by two
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